FiveThirtyEight's Nate Silver: Why Trump Is Polling Better, for Now

Donald Trump and Hillary Clinton are neck-and-neck, according to FiveThirtyEight, but it depends on how the polls are measured.

Nate Silver, founder and editor-in-chief of FiveThirtyEight, uses a polls-plus forecast which accounts for much of Trump's gain following the Republican National Convention last week and still gives Clinton a 60 percent chance of victory in November.

However, a polls-only forecast excluding the post-convention bump gives Clinton only a 53 percent chance of winning. If the election were held today, the data shows Trump would be the favorite, with a 55 percent chance of taking the White House.

The difference comes from multiple variables, such as including third-party candidates Gary Johnson of the Libertarian Party and the Green Party's Jill Stein, who are often excluded from polls, but when included usually draw more support away from Clinton than Trump.

According to Silver, most differences occur because of "how quickly the models adjust to new polling data." Clinton has been on a steady decline throughout July, so how should projections take that into account?

Going by analyses of previous campaigns, Silver and his staff found that a conservative "loess regression," which is a method of finding trends in data, produces a more accurate depiction further from the election, while an aggressive loess regression is better closer to Election Day.

Another problem is applying national polls on a state level. For instance, determining if Clinton will carry Pennsylvania is difficult if there haven't been any recent statewide polls to go by.

"Although there are other factors that matter around the margin, our models show better numbers for Trump mostly because they're more aggressive about detecting trends in polling data," Silver concludes.

"For the past couple of weeks — and this started before the conventions, so it's not just a convention bounce — there's been a strong trend away from Clinton, and toward Trump."